This invention generally relates to refrigerated mobile trailers, and is specifically concerned with a refrigerated trailer having condenser and radiator air outlets disposed in its roof panel for preventing waste to heat from recirculating through the condenser and radiator air inlets.
Refrigerated mobile trailers are well known in the prior art. Such trailers typically comprise a rectangular trailer body adapted to be hauled by a tractor, and a transportable refrigeration unit mounted over an opening in the front wall of the trailer body. The refrigeration unit is often powered by its own diesel engine. In order to discharge the waste heat generated by both the condenser of the refrigeration unit and the radiator of the internal combustion engine, the housing of the refrigeration unit includes at least one air inlet in its side or front panel, and air outlets variously located in its floor panel, side panel, and roof panel. A blower is disposed within the housing of the refrigeration unit for withdrawing air from the air inlet, and forcefully circulating it over both the condenser coil and the radiator of the engine. The circulated air is discharged out of the air outlets located in the floor, side, and roof of the refrigeration unit housing, which in turn carries away the waste heat generated by these components.
While such a refrigerated trailer design has generally proven itself to be effective for its intended purposes, the applicants have noted that such a design may fail to maintain a desired set point temperature within the trailer body when the ambient temperature is 120.degree. F. or higher. The applicants have further observed that such a failure is a result of two factors including (1) the lower high pressure cut-off temperature associated with newer, non-fluorocarbon refrigerants, and (2) the placement of the air outlet in the floor of the refrigeration unit housing.
It is well known that the cooling capacity of a refrigeration unit condenser is dependent upon the difference between the high pressure cut-off temperature of the refrigerant and the ambient temperature. Older fluorocarbon refrigerants, such as R-12 or R-502, had a high pressure cut-off temperature of 178.degree. F. Accordingly, the difference between this temperature and maximum ambient temperatures was almost always greater than 50.degree. F., thus providing substantial amount of condenser cooling capacity even when the ambient temperature reached very high levels on the order of 120.degree. F. Unfortunately, the high pressure cut-off temperature of newer, non-fluorocarbon refrigerants such as R-404 is only 149.degree. F., which substantially lowers the difference between the high pressure cut-off temperature and a possible ambient temperature of 120.degree. F. to only about 29.degree. F.
While such a 29.degree. F. difference still results in a cooling capacity capable of maintaining most refrigeration set points, the applicants have noted that the second factor, i.e., the placement of the air outlet on the refrigeration unit, can artificially lower this difference to only 19.degree. F. or less when the mobile trailer is stationary. Such conditions might arise when a loading or unloading operation is being conducted, or when the trailer is used to temporarily "warehouse" temperature sensitive goods. Under such circumstances, the column of hot air discharged from the floor-located air outlet is not laterally swept away (as it would be if the trailer were moving), but instead rises upward in a vertical direction, where it can be reintroduced into the side or front wall located air inlet of the refrigeration unit. Such undesirable recirculation of the hot air discharged from the air outlet can have the affect of artificially raising the temperature of the air surrounding the refrigeration unit 10.degree. F. or higher, thereby dropping the difference between the high pressure cut-off temperature of the refrigerant and high ambient temperatures to (120.degree. plus an additional 10.degree.) 19.degree. or less. With such a small difference between the high pressure cut-off temperature of 149.degree. F. and the artificially high ambient temperature of 130.degree. F., the condenser of the refrigeration unit cannot liquefy enough refrigerant for the expansion valve and evaporator assembly to maintain the refrigeration set point in the interior of the trailer. Under such conditions, prior art refrigerated trailers may be forced to operate at reduced capacities.
Clearly, there is a need for an improved refrigerated trailer design that is capable of maintaining refrigerated set point temperatures under all ambient air conditions, whether stationary or moving. Such a design must be compatible with the use of recently developed refrigerants, such as R-404, which do not relay upon atmosphere polluting fluoro or chlorocarbon compounds. Finally, it would be desirable if such a design was easily incorporated into existing trailers so that all the advantages of the design could be realized with only a minimal amount of structural modifications to existing trailers.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.